Process for producing porous film and porous film

ABSTRACT

A method of the invention produces a porous film by casting a polymer solution containing a polymer onto a substrate to form a film, and subjecting the film to phase conversion to thereby form a porous film. In the method, the polymer constituting the porous film has a surface tension Sa [mN/m], the substrate has a surface tension Sb [mN/m], and Sa and Sb satisfy the following condition: Sa−Sb≧−10. This method can produce a porous film having a high rate of hole area at its surfaces and having homogenous micropores from the surfaces to the core thereof. A porous film of the invention is a porous film having a large number of continuous micropores. The film has a thickness of 5 to 200 μm, has an average surface pore size A of 0.01 to 10 μm and an average rate of surface hole area C and has an average inside pore size B and an average rate of inside hole area D inside thereof, in which the ratio A/B of A to B is 0.3 to 3, and the ratio C/D of C to D is 0.7 to 1.5.

TECHNICAL FIELD

The present invention relates to porous films that are substantiallyfree from a skin layer (compact layer) on their surfaces and have alarge number of continuous micropores. The porous films can be used formembrane separation techniques such as microfiltration andseparation-concentration and can be used as a wide variety of substratematerials such as cell separators, electrolytic capacitors and circuitsubstrates by utilizing the properties of pores as intact or by chargingthe pores with a functional material.

BACKGROUND ART

Certain polymeric compounds such as amide-imide polymers, imidepolymers, sulfone polymers, fluorocarbon polymers and olefin polymersare known as materials for constituting porous films. Porous films areproduced from these materials, for example, by a phase conversiontechnique in which a mixture containing the polymeric compound is castas a film and the film is brought to a solidifying liquid. The resultingfilm produced by the phase conversion technique using the polymericcompound as a material, however, has a skin layer (compact layer) on itssurface and contains substantially no hole area (opening) or containssome openings with a low rate of hole area. Porous films using apolyimide, a kind of imide polymers, as a material and productionthereof, for example, are disclosed in Japanese Unexamined PatentApplication Publication No. 2001-67643, No. 2001-145826 and No.2000-319442. These films, however, must be produced using a controlmember for a solvent replacement rate, thereby require complicatedproduction processes and have insufficient rates of hole area andpermeability.

DISCLOSURE OF INVENTION

Accordingly, an object of the present invention is to provide a porousfilm having a high rate of hole area at its surfaces and containinghomogenous micropores overall from the surfaces to the inside thereof.

Another object of the present invention is to provide a method foreasily and conveniently producing the porous film.

After intensive investigations to achieve the objects, the presentinventors have found that a porous film having homogenous microporeswith a high rate of hole area even on a surface of the film which hasbeen in contact with a substrate can be prepared by casting a solutionmixture containing a polymer onto a substrate to form a film andsubjecting the film to phase conversion, in which the difference insurface tension between the polymer and the substrate is at a specificlevel or more. The present invention has been achieved based on thesefindings.

Specifically, the present invention provides a method for producing aporous film, including the steps of casting a polymer solutioncontaining at least one polymer onto a substrate to form a film; andsubjecting the film to phase conversion to thereby form a porous film,in which the polymer constituting the porous film has a surface tensionSa [mN/m], the substrate has a surface tension Sb [mN/m], and Sa and Sbsatisfy the following condition: Sa−Sb≧−10.

The method may produce a porous film by casting a solution mixture asthe polymer solution onto the substrate to form a film, and subjectingthe film to phase conversion by bringing the film to a solidifyingliquid to thereby form a porous film, in which the solution mixturecontains 8 to 25 percent by weight of a polymer component forconstituting the porous film, 10 to 50 percent by weight of awater-soluble polymer, 0 to 10 percent by weight of water and 30 to 82percent by weight of a water-soluble polar solvent. In addition oralternatively, the method may further include the steps of holding thecast film in an atmosphere at a relative humidity of 70% to 100% and atemperature of 15° C. to 90° C. for 0.2 to 15 minutes, and bringing thefilm to a solidifying liquid comprising a nonsolvent for the polymercomponent.

The present invention further provides a porous film having a largenumber of continuous micropores, in which the film has a thickness of 5to 200 μm, has an average surface pore size A of 0.01 to 10 μm and anaverage rate of surface hole area C and has an average inside pore sizeB and an average rate of inside hole area D, the ratio A/B of A to B isin the range of 0.3 to 3 and the ratio C/D of C to D is in the range of0.7 to 1.5.

In addition, the present invention provides a porous film having a largenumber of continuous micropores, in which the film has a thickness of 5to 200 μm, has an average pore size A¹ of 0.01 to 10 μm at one surface,an average pore size A² of 0.01 to 10 μm, at the other surface, anaverage rate of hole area C¹ of 48% or more at one surface, and anaverage rate of hole area C² of 48% or more at the other surface, theratio A¹/A² of A¹ to A² is in the range of 0.3 to 3, and the ratio C¹/C²of C¹ to C² is in the range of 0.7 to 1.5.

The production method of the present invention can easily andconveniently produce a porous film having homogenous micropores with animproved rate of hole area even at a surface which has been in contactwith a substrate, since a solution mixture containing a polymercomponent forms a satisfactory phase separation structure on thesubstrate. The porous film of the present invention can be used formembrane separation techniques such as microfiltration andseparation-concentration and can be used as a wide variety of substratematerials such as cell separators, electrolytic capacitors and circuitsubstrates by charging the pores with a functional material.

BEST MODE FOR CARRYING OUT THE INVENTION

The method of the present invention produces a porous film by casting apolymer solution containing a polymer component as a material forconstituting the porous film onto a substrate to form a film andsubjecting the film to phase conversion.

The polymer component includes, but is not limited to, polymers such asamide-imide polymers, imide polymers, amide polymers, sulfone polymers,cellulosic polymers, acrylic polymers, fluorocarbon polymers andolefinic polymers. Among them, polymer components being soluble in awater-soluble polar solvent and capable of forming a film by phaseconversion are preferred. Examples of such preferred polymers areamide-imide polymers, imide polymers, polyethersulfones, polysulfones,acrylic polymers and cellulose acetate. Each of these polymer componentscan be used alone or in combination.

Examples of the substrate are glass plate; plastic sheets made of, forexample, polyolefins such as polyethylenes, polypropylenes andpolymethylpentenes, nylons (polyamides), polyesters such aspoly(ethylene terephthalate)s (PET), polycarbonates, styrenic resins,fluorocarbon resins such as polytetrafluoroethylenes (PTFE) andpoly(vinylidene fluoride)s (PVDF), vinyl chloride resins and otherresins; metal plates such as stainless steel plate and aluminum plate.The substrate may be a composite plate or sheet comprising a surfacelayer and a core, the surface layer and the core comprising differentmaterials from each other.

A main feature of the present invention is to produce a porous film byusing a polymer and a substrate, in which the polymer constituting theporous film has a surface tension Sa [mN/m (=dyn/cm)], the substrate hasa surface tension Sb [mN/m (=dyn/cm)], and Sa and Sb satisfy thefollowing condition: Sa—Sb≧−10. If the substrate is a composite platecomprising a surface layer and a core comprising different materialsfrom each other, only the material to form a contact surface with thepolymer has to have a surface tension satisfying the requirement. If thedifference (Sa−Sb) is less than −10, the resulting film has a low rateof surface hole area and cannot be practically used, since the polymercoagulates at the interface between the polymer and the substrate tothereby form a compact phase.

By using such a polymer and a substrate satisfying the above-specifiedcondition, the solution mixture containing the polymer undergoes phaseseparation to form an islands-in-sea structure on the substrate, whichresults in micropores of the resulting film. This specifically yields aporous film having a high rate of hole area especially at a surfacewhich has been in contact with the substrate (hereinafter may bereferred to as “substrate-side surface of the film”). The difference(Sa−Sb) is preferably more than 0, more preferably 3 or more, furtherpreferably 7 or more, and specifically preferably 13 or more for betteropening (porosity), since the polymer coagulated as a result of phaseconversion cannot wet the surface of the substrate and is rejected undersuch conditions. The upper limit of the difference (Sa−Sb) is notspecifically limited and can be, for example, about 100.

The polymer solution to be cast for use in the present invention is, forexample, preferably a solution mixture comprising 8 to 25 percent byweight of a polymer component for constituting the porous film, 10 to 50percent by weight of a water-soluble polymer, 0 to 10 percent by weightof water and 30 to 82 percent by weight of a water-soluble polarsolvent. An excessively low concentration of the polymer component mayinvite a reduced strength of the film. An excessively high concentrationof the polymer component may decrease the porosity. The water-solublepolymer is added to form a homogenous spongy porous structure inside thefilm. An excessively low concentration of the water-soluble polymer mayinvite giant voids with a size exceeding, for example, 10 μm inside thefilm and invite a decreased uniformity of the pores. An excessively highconcentration of the water-soluble polymer may invite a decreasedsolubility. If the concentration exceeds 50 percent by weight, the filmmay have a decreased strength.

Examples of the water-soluble polar solvent are dimethyl sulfoxide,N,N-dimethylformamide, N,N-dimethylacetamide (DMAc),N-methyl-2-pyrrolidone (NMP), 2-pyrrolidone, and mixtures of thesesolvents. A solvent corresponding to the chemical structure of thepolymer used as the polymer component and capable of dissolving thepolymer (a good solvent for the polymer component) can be used. Each ofthese solvents can be used alone or in combination.

The water-soluble polymer and water are added so as to control the phaseseparation structure upon casting to thereby form a spongy, porous filmstructure. Examples of the water-soluble polymer are polyethyleneglycol, polyvinylpyrrolidones, poly(ethylene oxide)s, poly(vinylalcohol)s, poly(acrylic acid)s, polysaccharides, and derivatives ofthese polymers. Each of these water-soluble polymers can be used aloneor in combination. Among them, polyvinylpyrrolidones are preferred foryielding satisfactorily continuous micropores in the film. The molecularweight of the water-soluble polymer is preferably 1000 or more, morepreferably 5000 or more, specifically preferably 10000 or more, andespecialy preferably from about 10000 to about 200000, for forming asatisfactorily porous structure. The size of voids can be controlled bychanging the amount of water. The size can increase with an increasingamount of water.

Homogenous micropores can be formed by casting a solution mixture havingthe above composition as the polymer solution onto a substrate to form afilm, and subjecting the film to phase conversion by bringing the filmto a solidifying liquid.

It is preferred that the cast film is held in an atmosphere at relativehumidity of 70% to 100% and temperatures of 15° C. to 90° C. for 0.2 to15 minutes, and the film is brought to a solidifying liquid comprising anonsolvent for the polymer component. The cast film is more preferablyheld at relative humidity of 90% to 100% and temperatures of 30° C. to80° C., and specifically preferably at relative humidity of about 100%(e.g., 95% to 100%) and temperatures of 40° C. to 70° C. If the moisturecontent in the atmosphere is less than the above-specified range, theresulting film may have an insufficient rate of hole area.

By keeping the cast film under the above-mentioned condition, the filmcan have an increased rate of hole area specifically on a surfaceopposite to the substrate-side surface of the film (hereinafter may bereferred to as “air-side surface of the film”). The rate of hole area isincreased provably because water (moisture) migrates from the surfaceinto the core of the film and efficiently accelerates the phaseseparation of the solution mixture by holding the cast film under ahumidified condition.

The solidifying liquid for use in the phase conversion can be anysolvent that serves to solidify the polymer component and is selectedcorresponding to the type of the polymer component. Examples of thesolidifying liquid are water; alcohols including monohydric alcoholssuch as methanol and ethanol, and polyhydric alcohols such as glycerol;water-soluble polymers such as polyethylene glycol; and mixture of thesesubstances.

The method of the present invention can produce a porous film havinghomogenous micropores with a high rate of hole area. The porous filmproduced by the method of the present invention will be illustratedbelow.

The thickness of the porous film is, for example, from 5 to 200 μm,preferably from 10 to 100 μm and more preferably from 20 to 80 μm. Anexcessively small thickness may invite insufficient mechanical strengthof the film, and an excessively large thickness may fail to control thepore size distribution uniformly.

The average pore size of micropores in the porous film, i.e., theaverage pore size at the film surface, may vary depending on the use ofthe film and is generally from 0.01 to 10 μm, and preferably from 0.05to 5 μm. An excessively small size may invite a decreased permeationcapability of the film, and an excessively large size may invite adecreased efficiency in separation and concentration. A functionalmaterial, if used, is preferably charged into the pores with aresolution on the order of submicrons to microns, and theabove-specified average pore size is preferred for this purpose. If theaverage pore size is excessively small, the functional material may notbe charged. If it is excessively large, the control on the order ofsubmicrons to microns may be difficult. The maximum pore size at thefilm surface is preferably 15 μm or less.

The average rate of hole area inside the porous film (rate of insidehole area; porosity) is, for example, from 30% to 80%, preferably from40% to 80% and more preferably from 45% to 80%. An excessively lowporosity may invite insufficient permeation capability of the film orinsufficient action of the functional material, if charged. In contrast,an excessively high porosity may invite deteriorated mechanicalstrength. The average rate of hole area at the film surface (rate ofsurface hole area) is, for example, 48% or more (e.g., 48% to 80%) andpreferably from about 60% to about 80%. An excessively low rate ofsurface hole area may invite insufficient permeation capability of thefilm or insufficient action of the functional material, if charged. Incontrast, an excessively high rate of surface hole area may invitedecreased mechanical strength.

The continuity of the micropores of the film can be indicated, forexample, in terms of a Gurley permeability as a gas permeability and bya pure-water permeation rate. The Gurley permeability of the porous filmis, for example, from 0.2 to 29 seconds per 100 cc, preferably from 1 to25 seconds per 100 cc, and specifically preferably from 1 to 18 secondsper 100 cc. If the Gurley permeability exceeds the above-specifiedrange, the film may have insufficient permeability upon actual useand/or the functional material may not be sufficiently charged into thepores and may not sufficiently exhibit its function. If the Gurleypermeability is less than the above-specified range, the film may havedeteriorated mechanical strength. The pure-water permeation rate is, forexample, from 1.3×10⁻⁹ to 1.1×10⁻⁷ m·sec⁻¹·Pa^(−1 [=8) to 700liter/(m²·min·atm)], preferably from 3.3×10⁻⁹ to 1.1×10⁻⁷ m·sec⁻¹·Pa⁻¹[=20 to 700 liter/(m²·min·atm)] and more preferably from 4.9×10⁻⁹ to8.2×10⁻⁸ m·sec⁻¹·Pa^(−1 [=30) to 500 liter/(m²·min·atm)]. If thepure-water permeation rate is less than the above-specified range, thefilm may have insufficient permeability upon actual use and/or thefunctional material may not be sufficiently charged into the pores andmay not sufficiently exhibit its function. If it exceeds theabove-specified range, the film may have deteriorated mechanicalstrength.

The porous film is, in one embodiment, preferably a porous film having alarge number of continuous micropores, wherein the film has a thicknessof 5 to 200 μm, has an average surface pore size A of 0.01 to 10 μm andan average rate of surface hole area C and has an average inside poresize B and an average rate of inside hole area D, in which the ratio A/Bof A to B is in the range of 0.3 to 3, and the ratio C/D of C to D is inthe range of 0.7 to 1.5.

The ratio A/B of the average surface pore size A to the average insidepore size B and the ratio C/D of the average rate of surface hole area Cto the average rate of inside hole area D are preferably from 0.5 to 2and from 0.75 to 1.4, respectively, and more preferably from 0.6 to 1.5and from 0.8 to 1.3, respectively. If these ratios are excessively low,the film may have a deteriorated permeation capability and/or maycontain the functional material insufficiently charged. If they areexcessively high, the film may have a deteriorated separation capabilityand/or may contain the functional material heterogeneously charged.

In another embodiment, the porous film is preferably a porous filmhaving a large number of continuous micropores, wherein the film has athickness of 5 to 200 μm, has an average pore size A¹ of 0.01 to 10 μmat one surface (e.g., the substrate-side surface), an average pore sizeA² of 0.01 to 10 μm at the other surface (e.g., the air-side surface),an average rate of hole area C¹ of 48% or more at one surface, and anaverage rate of hole area C² of 48% or more at the other surface, inwhich the ratio A¹/A² of A¹ to A² is in the range of 0.3 to 3, and theratio C¹/C² of C¹ to C² is in the range of 0.7 to 1.5.

The ratio A¹/A² of the average pore size at one surface A¹ to theaverage pore size at the other surface A², and the ratio C¹/C² of theaverage rate of hole area at one surface C¹ to the average rate of holearea at the other surface C2 are preferably from 0.5 to 2 and from 0.75to 1.4, respectively, and more preferably from 0.6 to 1.5 and from 0.8to 1.3, respectively. If these ratios are excessively low, the film mayhave a deteriorated permeation capability and/or may contain thefunctional material insufficiently charged. If they are excessivelyhigh, the film may have a deteriorated separation capability and/or maycontain the functional material heterogeneously charged.

The pore size, porosity, gas permeability and rate of hole area of themicropores of the porous film can be controlled at desired levels bysuitably selecting, for example, the substrate to be used, the type andamount of the water-soluble polymer, the amount of water, the humidity,temperature and time period in casting.

The method of the present invention can easily produce a porous filmhaving, in terms of average pore size and average rate of hole area,ratios of the surface to the inside and the ratios of the substrate-sidesurface to the air-side surface within the above-specified ranges.

EXAMPLES

The present invention will be illustrated in further detail withreference to several examples below, which are never intended to limitthe scope of the invention. The surface tension and the properties ofthe resulting films were determined in the following manner. The resultsare shown in Table 1 (Table 1-1, Table 1-2), wherein “Sa−Sb” means thedifference between the surface tension of a polymer constituting thefilm Sa and the surface tension of the substrate Sb. The symbol “-”means that pores have too indefinite shapes to determine the pore size.

Surface Tension

The surface tensions listed in POLYMER HANDBOOK (THIRD EDITION, JOHNWILEY & SONS) and Handbook of Chemical Engineering (revised 5th Ed.,MARUZEN CO., LTD.) were used. The surface tension of a substance notlisted in these books was determined by using a homogenous film of asample polymer alone (or a polymer blend) according to JapaneseIndustrial Standards (JIS) K 6768. The surface tensions listed in theexamples and determined by the latter method are indicated as“measured”.

Gas Permeability

The gas permeability of a sample film was determined using a Gurley'sDensometer available from YOSHIMITSU according to the method describedin JIS P 8117. In the procedure, the Gurley's Densometer used is ofone-tenth scale in measuring area as compared with the standard, and themeasured value was converted into a standard Gurley permeabilityaccording to Appendix 1 of JIS P 8117.

Pure-Water Permeation Rate

The pure-water permeation rate was determined using a filter STIRREDULTRAFILTRATION CELLS MODELS 8200 available from Amicon with apermeation area of 28.7 cm². In the determination, the resistance on thepermeation side was eliminated as far as possible by placing a filterpaper instead of a spacer at the permeation side. The pressure was setat 0.5 kg/cm² and the measured value was converted. The temperature wasset at 25° C. in determination.

Average Surface Pore Size A

The areas of arbitrary thirty or more pores at a surface of a samplefilm were determined, and the average thereof was defined as an averagepore area S_(ave). The average pore area was converted into a pore size(pore diameter) according to the following equation, assuming that thepores are perfect circles in profile, and the converted pore size wasdefined as the average pore size. In the equation, the symbol “π”represents the ratio of the circumference of a circle to its diameter.Surface Average Pore Size A=2×(S _(ave)/π)^(1/2)

Average Inner Pore Size B

A sample film was broken at temperatures of liquid nitrogen, and asection of the film was exposed. When the sample film was not brokenaccording to this method, the film was wetted with water and was thenbroken at temperatures of liquid nitrogen to thereby expose the sectionthereof. The average pore size was determined in the same way as theabove average surface pore size by using the section of the film as asample for electron microscopy.

Maximum Surface Pore Size

Arbitrary five 20-μm square points were selected from an electronmicrograph of the surface of a sample film. The diameters of pores atthe five points were converted into pore sizes according to thefollowing equation, assuming that the pores are perfect circles, and thelargest one was defined as the maximum pore size. In the equation,S_(max) is the largest area (maximum area) of measured pores, and symbol“π” represents the ratio of the circumference of a circle to itsdiameter.Pore Size=2×(S _(max)/π)^(1/2)

Maximum Inner Pore Size

A sample film was broken at temperatures of liquid nitrogen, and asection of the film was exposed. When the sample film was not brokenaccording to this method, the film had been wetted with water and wasbroken at temperatures of liquid nitrogen to thereby expose the sectionthereof. The maximum pore size was then determined in the same way asthe above maximum surface pore size by using the section of the film asa sample for electron microscopy.

Average Rate of Surface Hole Area C

An arbitrary 20-μm square area was selected from an electron micrographof the surface of a sample film. The ratio of the total area of pores inthe selected area to the total area was determined by calculation. Thisprocedure was carried out at arbitrary five points, and the averagethereof was determined as the average rate of surface hole area.

Average Rate of Inner Hole Area D (Porosity)

The average rate of inner hole area of a sample film was determinedaccording to the following equation, wherein V is the volume of thefilm; W is the weight of the film; and ρ is the density of a materialfor the film. The densities of a poly(amide imide), a polyethersulfoneand a blend of a poly(amide imide) and a polyethersulfone used inExample 6 were 1.45 (g/cm³), 1.37 (g/cm³) and 1.43 (g/cm³),respectively.Average Rate of Inner Hole Area D (%)=100−100×W/(ρ·W)

The average pore sizes, maximum pore sizes and average rates of holearea were determined only on a most front micropore in the electronmicrophotograph, and the other micropores in the electron micrographwere excluded in determination.

Example 1

A solution of an amide-imide polymer “VYLOMAX HR11NN” (trade name of aproduct of Toyobo Co., Ltd.) having a measured surface tension as thepolymer alone of 42 mN/m (=dyn/cm), a solid content of 15 percent byweight, containing NMP as a solvent and having a viscosity as a solutionof 20 dPa·s at 25° C. was used. A total of 30 parts by weight of apolyvinylpyrrolidone having a molecular weight of 5×10⁴ as awater-soluble polymer was added to 100 parts by weight of the solutionto thereby yield a composition for film formation. The composition wascast onto a Teflon (registered trademark) substrate having a surfacetension of 24 mN/m (=dyn/cm) using a film applicator at a temperature of30° C. and relative humidity of 80%. Immediately after casting, the castfilm on the substrate was held in a container at a temperature of 45° C.and humidity of about 100% for four minutes. The film was solidified byimmersing in water, was dried and thereby yielded a porous film. Thedistance (gap) between the film applicator and the Teflon (registeredtrademark) substrate in casting was set at 127 μm, and the film had athickness of about 50 μm.

The structure of the resulting film was observed. A surface of the filmwhich had been in contact with the substrate upon casting(substrate-side surface of the film) contained pores having an averagepore size A¹ of about 0.9 μm, a maximum pore size of 2.5 μm and anaverage rate of hole area C¹ of about 65%. Another surface of the filmwhich had not been in contact with the substrate upon casting (air-sidesurface of the film) contained pores having an average pore size A² ofabout 1.1 μm, a maximum pore size of 2.7 μm and an average rate of holearea C² of about 70%. The inside of the film was substantiallyhomogenous and entirely contained continuous micropores having anaverage pore size B of about 1.0 μm and a maximum pore size of about 1.8μm with an average rate of inner hole area D of 70%. The permeationcapability of the film was determined to find that the film had superiorpermeation capabilities in terms of a Gurley permeability of 9.5 secondsand a pure-water permeation rate of 9.8×10⁻⁹ m·sec⁻¹·Pa⁻¹ [=60liter/(m²·min·atm at 25° C.)].

Example 2

A film was prepared by the procedure of Example 1, except for using apolypropylene substrate having a surface tension of 29 mN/m (=dyn/cm) asa substrate for casting instead of the Teflon (registered trademark)substrate.

The structure of the resulting film was observed. The substrate-sidesurface of the film contained pores having an average pore size A¹ ofabout 0.7 μm, a maximum pore size of 1.8 μm and an average rate of holearea C¹ of about 50%. The air-side surface of the film contained poreshaving an average pore size A2 of about 1.0 μm, a maximum pore size of2.5 μm and an average rate of hole area C² of about 70%. The inside ofthe film was substantially homogenous and entirely contained continuousmicropores having an average pore size B of about 1.0 μm and a maximumpore size of about 2.0 μm with an average rate of inner hole area D ofabout 70%. The permeation capability of the film was determined to findthat the film had superior permeation capabilities in terms of a Gurleypermeability of 10.0 seconds and a pure-water permeation rate of9.0×10⁻⁹ m·sec⁻¹·Pa⁻¹ [=55 liter/(m²·min·atm at 25° C.)].

Example 3

A film was prepared by the procedure of Example 1, except for using apoly(ethylene terephthalate) (PET) sheet having a measured surfacetension of 39 mN/m (=dyn/cm) (a product of DuPont Teijin Films, Ltd.;Type S) as a substrate for casting instead of the Teflon (registeredtrademark) substrate.

The structure of the resulting film was observed. The substrate-sidesurface of the film contained pores having an average pore size A¹ ofabout 0.9 μm, a maximum pore size of 2.5 μm and an average rate of holearea C¹ of about 70%. The air-side surface of the film contained poreshaving an average pore size A² of about 1.0 μm, a maximum pore size of2.7 μm and an average rate of hole area C² of about 70%. The inside ofthe film was substantially homogenous and entirely contained continuousmicropores having an average pore size B of about 1.0 μm and a maximumpore size of about 2.0 μm with an average rate of inner hole area D ofabout 70%. The permeation capability of the film was determined to findthat the film had superior permeation capabilities in terms of a Gurleypermeability of 10.0 seconds and a pure-water permeation rate of9.0×10⁻⁹ m·sec⁻¹·Pa⁻¹ [=55 liter/(m²·min·atm at 25° C.)].

Example 4

A film was prepared by the procedure of Example 2, except for using amixture of 15 parts by weight of a polyethersulfone (a product ofSumitomo Chemical Co., Ltd. under the trade name of “5200 P”) having ameasured surface tension of 46 mN/m (=dyn/cm), 10 parts by weight of apolyvinylpyrrolidone having a molecular weight of 36×10⁴ and 75 parts byweight of NMP as a composition for film formation.

The structure of the resulting film was observed. The substrate-sidesurface of the film contained pores having an average pore size A¹ ofabout 1.3 μm, a maximum pore size of 2.5 μm and an average rate of holearea C¹ of about 65%. The air-side surface of the film contained poreshaving an average pore size A² of about 0.8 μm, a maximum pore size of1.7 μm and an average rate of hole area C² of about 50%. The inside ofthe film was substantially homogenous and entirely contained continuousmicropores having an average pore size B of about 2.0 μm and a maximumpore size of 3.0 μm with an average rate of inner hole area D of about70%. The permeation capability of the film was determined to find thatthe film had superior permeation capabilities in terms of a Gurleypermeability of 29 seconds and a pure-water permeation rate of 3.3×10⁻⁹m·sec⁻¹·Pa⁻¹ [=20 liter/(m²·min·atm at 25° C.)].

Example 5

A film was prepared by the procedure of Example 4, except for using apoly(ethylene terephthalate) (PET) sheet having a measured surfacetension of 39 mN/m (=dyn/cm) (a product of DuPont Teijin Films, Ltd.;Type S) as a substrate for casting instead of the polypropylenesubstrate.

The structure of the resulting film was observed. The substrate-sidesurface of the film contained pores having an average pore size A¹ ofabout 2.3 μm, a maximum pore size of 3.6 μm and an average rate of holearea C¹ of about 65%. The air-side surface of the film contained poreshaving an average pore size A² of about 0.8 μm, a maximum pore size of1.7 μm and an average rate of hole area C² of about 50%. The inside ofthe film was substantially homogenous and entirely contained continuousmicropores having an average pore size B of about 2.0 μm and a maximumpore size of 5.1 μm with an average rate of inner hole area D of about70%. The permeation capability of the film was determined to find thatthe film had superior permeation capabilities in terms of a Gurleypermeability of 27 seconds and a pure-water permeation rate of 3.9×10⁻⁹m·sec⁻¹·Pa⁻¹ [=24 liter/(m²·min·atm at 25° C.)].

Example 6

Composition A was prepared by mixing 100 parts by weight of a solutionof an amide-imide polymer “VYLOMAX HR11NN” (trade name of a product ofToyobo Co., Ltd.) having a measured surface tension as the polymer aloneof 42 mN/m (=dyn/cm), a solid content of 15 percent by weight,containing NMP as a solvent and having a viscosity as a solution of 20dpa·s at 25° C. and 25 parts by weight of a polyvinylpyrrolidone havinga molecular weight of 5×10⁴. Composition B was prepared by adding 85parts by weight of NMP to 15 parts by weight of a polyethersulfone (aproduct of Sumitomo Chemical Co., Ltd. under the trade name of “5200 P”)having a measured surface tension of 46 mN/m (=dyn/cm) to yield amixture, and adding 25 parts by weight of a polyvinylpyrrolidone havinga molecular weight of 5×10⁴ to 100 parts by weight of the mixture.

A film was prepared by the procedure of Example 2, except for using, asa composition for film formation, a 3:1 (by weight) mixture of anamide-imide polymer and a polyethersulfone, i.e., a 3:1 (by weight)mixture of Composition A and Composition B, as a blend polymer having ameasured surface tension of 45 mN/m (=dyn/cm).

The structure of the resulting film was observed. The substrate-sidesurface of the film contained pores having an average pore size A¹ ofabout 0.9 μm, a maximum pore size of 1.8 μm and an average rate of holearea C¹ of about 70%. The air-side surface of the film contained poreshaving an average pore size A² of about 2.0 μm, a maximum pore size of4.4 μm and an average rate of hole area C² of about 70%. The inside ofthe film was substantially homogenous and entirely contained continuousmicropores having an average pore size B of about 2.0 μm and a maximumpore size of about 3.0 μm with an average rate of inner hole area D ofabout 70%. The permeation capability of the film was determined to findthat the film had superior permeation capabilities in terms of a Gurleypermeability of 9.3 seconds and a pure-water permeation rate of 1.1×10⁻⁸m·sec⁻¹·Pa⁻¹ [=24 liter/(m²·min·atm at 25° C.)].

Comparative Example 1

A film was prepared by the procedure of Example 1, except for using aglass substrate having a surface tension of 100 mN/m (=dyn/cm) as asubstrate for casting instead of the Teflon (registered trademark)substrate.

The structure of the resulting film was observed. The substrate-sidesurface of the film contained pores having an average pore size A¹ ofabout 0.3 μm, a maximum pore size of 0.6 μm and an average rate of holearea C¹ of about 40%. The air-side surface of the film contained poreshaving an average pore size A² of about 1.0 μm, a maximum pore size of2.5 μm and an average rate of hole area C² of about 70%. The inside ofthe film was substantially homogenous and entirely contained continuousmicropores having an average pore size B of about 1.0 μm and a maximumpore size of about 2.0 μm with an average rate of inner hole area D ofabout 70%. These results show that the film has micropores at thesubstrate-side surface thereof having dimensions and a rate of hole areasmaller than those of micropores at the air-side surface and inside thefilm, and that the film as a whole lacks uniformity.

Comparative Example 2

A film was prepared by the procedure of Example 1, except for using analuminum substrate having a surface tension of 914 mN/m (=dyn/cm) as asubstrate for casting instead of the Teflon (registered trademark)substrate.

The structure of the resulting film was observed. The substrate-sidesurface of the film contained pores having too irregular shapes todetermine the average pore size A¹. The average rate of hole area C¹ wasestimated as 10% or less. The air-side surface of the film containedpores having an average pore size A² of about 1.3 μm, a maximum poresize of 2.7 μm and an average rate of hole area C² of about 70%. Theinside of the film was substantially homogenous and entirely containedcontinuous micropores having an average pore size B of about 1.2 μm anda maximum pore size of about 2.2 μm with an average rate of inner holearea D of about 70%. These results show that the substrate-side surfaceof the film has a low rate of hole area and has peculiar dimensions ascompared with the air-side surface and the inside of the film, and thatthe film as a whole lacks uniformity.

Comparative Example 3

A film was prepared by the procedure of Example 4, except for using aglass substrate having a surface tension of 100 mN/m (=dyn/cm) as asubstrate for casting.

The structure of the resulting film was observed. The substrate-sidesurface of the film contained pores having an average pore size A¹ ofabout 1.2 μm, a maximum pore size of 2.0 μm and an average rate of holearea C¹ of 10% or less. The air-side surface of the film contained poreshaving an average pore size A² of about 0.8 μm, a maximum pore size of1.9 μm and an average rate of hole area C² of about 50%. The inside ofthe film entirely contained continuous micropores having an average poresize B of about 2.0 μm and a maximum pore size of about 3.5 μm with anaverage rate of inner hole area D of about 70%. These results show thatthe film has micropores at the substrate-side surface thereof having alow rate of hole area and that the film as a whole lacks uniformity.

Comparative Example 4

A film was prepared by the procedure of Example 4, except for using analuminum substrate having a surface tension of 914 mN/m (=dyn/cm) as asubstrate for casting instead of the Teflon (registered trademark)substrate.

The structure of the resulting film was observed. The substrate-sidesurface of the film contained pores having too irregular shapes todetermine the average pore size A¹. The average rate of hole area C¹ wasestimated as 10% or less. The air-side surface of the film containedpores having an average pore size A² of about 0.9 am, a maximum poresize of 2.1 μm and an average rate of hole area C² of about 50%. Theinside of the film entirely contained continuous micropores having anaverage pore size B of about 2.2 μm and a maximum pore size of about 3.6μm with an average rate of inner hole area D of about 70%. These resultsshow that the film has micropores at the substrate-side surface thereofhaving dimensions and a rate of hole area smaller than those ofmicropores at the air-side surface and inside the film, and that thefilm as a whole lacks uniformity. TABLE 1-1 Substrate-side surfaceAir-side surface Inside Average Average Average Average Maximum rate ofAverage Maximum rate of Average Maximum rate of pore size pore hole porepore hole pore pore hole Sa-Sb A¹ size area C¹ size A² size area C² sizeB size area D [dyn/cm] [μm] [%} [μm] [%} [μm] [%} Ex. 1 18 0.9 2.5 651.1 2.7 70 1.0 1.8 70 Ex. 2 13 0.7 1.8 50 1.0 2.5 70 1.0 2.0 70 Ex. 3 30.9 2.5 70 1.0 2.7 70 1.0 2.0 70 Ex. 4 17 1.3 2.5 65 0.8 1.7 50 2.0 3.070 Ex. 5 7 2.3 3.6 65 0.8 1.7 50 2.0 5.1 70 Ex. 6 16 0.9 1.8 70 2.0 4.470 2.0 3.0 70 Com. Ex. 1 −58 0.3 0.6 40 1.0 2.5 70 1.0 2.0 70 Com. Ex. 2−872 — — <10 1.3 2.7 70 1.2 2.2 70 Com. Ex. 3 −54 1.2 2.0 <10 0.8 1.9 502.0 3.5 70 Com. Ex. 4 −868 — — <10 0.9 2.1 50 2.2 3.6 70

TABLE 1-2 Surface/Inside Substrate-side/Air-side Ratio of average rateof Ratio of Ratio of Ratio of average pore size hole area average poreaverage rate Surface-side Air-side Surface-side Air-side size of holearea A¹/B A²/B C¹/D C²/D A¹/A² C¹/C² Ex. 1 0.9 1.1 0.93 1.0 0.82 0.93Ex. 2 0.7 1.0 0.71 1.0 0.70 0.71 Ex. 3 0.9 1.0 1.0 1.0 0.90 1.0 Ex. 40.65 0.4 0.93 0.71 1.63 1.3 Ex. 5 1.15 0.4 0.93 0.71 2.88 1.3 Ex. 6 0.451.0 1.0 1.0 0.45 1.0 Com. Ex. 1 0.3 1.0 0.57 1.0 0.3 0.57 Com. Ex. 2 —1.1 <0.14 1.0 — — Com. Ex. 3 0.6 0.4 <0.14 0.71 1.5 — Com. Ex. 4 — 0.41<0.14 0.71 — —

INDUSTRIAL APPLICABILITY

The porous films of the present invention can be used for membraneseparation techniques such as microfiltration andseparation-concentration and can be used as a wide variety of substratematerials such as cell separators, electrolytic capacitors and circuitsubstrates by charging the pores with a functional material.

1. A method for producing a porous film, comprising the steps of castinga polymer solution comprising a polymer onto a substrate to form a film;and subjecting the film to phase conversion to thereby form a porousfilm, wherein the polymer constituting the porous film has a surfacetension Sa [mN/m], wherein the substrate has a surface tension Sb[mN/m], and wherein Sa and Sb satisfy the following condition:Sa−Sb≧−10.
 2. The method for producing a porous film according to claim1, further comprising the steps of casting a solution mixture as thepolymer solution onto the substrate to form a film, and subjecting thefilm to phase conversion by bringing the film to a solidifying liquid tothereby form a porous film, the solution mixture comprising 8 to 25percent by weight of a polymer component for constituting the porousfilm, 10 to 50 percent by weight of a water-soluble polymer, 0 to 10percent by weight of water and 30 to 82 percent by weight of awater-soluble polar solvent.
 3. The method for producing a porous filmaccording to one of claims 1 and 2, further comprising the steps ofholding the cast film in an atmosphere at a relative humidity of 70% to100% and a temperature of 15° C. to 90° C. for 0.2 to 15 minutes, andbringing the film to a solidifying liquid comprising a nonsolvent forthe polymer component.
 4. A porous film having a large number ofcontinuous micropores, wherein the film has a thickness of 5 to 200 μm,has an average surface pore size A of 0.01 to 10 μm and an average rateof surface hole area C and has an average inside pore size B and anaverage rate of inside hole area D, wherein the ratio A/B of A to B isin the range of 0.3 to 3, and wherein the ratio C/D of C to D is in therange of 0.7 to 1.5.
 5. A porous film having a large number ofcontinuous micropores, wherein the film has a thickness of 5 to 200 μm,has an average pore size A¹ of 0.01 to 10 μm at one surface, an averagepore size A² of 0.01 to 10 μm at the other surface, an average rate ofhole area C¹ of 48% or more at one surface, and an average rate of holearea C² of 48% or more at the other surface, wherein the ratio A¹/A² ofA¹ to A² is in the range of 0.3 to 3, and wherein the ratio C¹/C² of C¹to C² is in the range of 0.7 to 1.5.